The use of driveline retarders as a means for applying braking torque to a vehicle transmission is well known. Typically, such driveline retarders employ a rotary member connected to the hub of a transmission output shaft that is operative to circulate fluid between vane members on a stationary housing. The fluid circulation results in power absorption and retardation of the rotational speed of the transmission output shaft. A reduction in vehicle speed typically results from such action, assuming that the roadway or other supportive surface of the vehicle demonstrates a frictional coefficient of sufficient magnitude to support such braking torque. The driveline retarder is especially suited to slow or control the speed of a vehicle so equipped when traversing steep grades.
It is generally known to employ a solenoid valve in hydrodynamic-type driveline retarders to function as a pressure control valve under electro-hydraulic solenoid modulation as controlled by an appropriate electronic control unit such as a dedicated microprocessor or the like. The duty cycle at which the solenoid valve is actuated or modulated regulates the driveline retarder capacity or the load applied to the output shaft of the transmission and, accordingly, controls the braking activity of the driveline retarder. The mechanical structure and control architecture of the previously known driveline retarders are generally adaptable for implementation in accordance with the present invention.